1. Field of the Invention
The invention relates to a method for forming a signal sequence which is to be transmitted in particular for the purpose of synchronization of at least two transmission units, and to a method for determining this signal sequence, and corresponding transmitting and receiving units.
2. Description of the Related Art
In signal transmission systems, such as mobile radio systems, it is necessary for one of the communication partners (a first transmission unit) to identify specific defined signals which are transmitted by another communication partner (second transmission unit). These may be, for example, so-called synchronization bursts (synchronization radio blocks) for synchronization of two synchronization partners, such as radio stations, or may be so-called access bursts.
In order to detect and to identify such received signals reliably against the background noise, it is known for the received signal to be continuously correlated with a predetermined signal sequence for a defined period of time, and for the correlation sum to be formed over the time period of the predetermined signal sequence. That region of the received signal which produces a maximum correlation sum corresponds to the sought signal. The synchronization signal from the base station in a digital mobile radio system is preceded, for example, by a signal sequence as a so-called training sequence, which is detected or determined in the manner just described in the mobile station, by correlation with the stored signal sequence. The mobile stations can thus be synchronized to the base station.
Correlation calculations of such a type are also required in the base station, for example for Random Access CHannel (RACH) detection. Furthermore, a correlation calculation is also carried out in order to establish the channel impulse response and the signal delay times of received signal bursts.
The correlation sum is in this case calculated as follows:
  Sm  =            ∑              i        =        0                    n        -        1              ⁢                  E        ⁡                  (                      i            +            m                    )                    *              K        ⁡                  (          i          )                    where E(i) is a received signal sequence derived from the received signal and K(i) is the predetermined signal sequence, with i running from 0 to n−1. The correlation sum Sm is calculated successively for a number of signal sequences E(i) at different times and obtained from the received signal, and the maximum value of Sm is then established. If k successive correlation sums are calculated, then the calculation effort comprises k*n operations, with a multiplication and addition together being counted as one operation.
The calculation of the correlation sums is thus highly complex and, particularly for real-time applications such as voice communication or video telephony, or in CDMA systems, requires powerful and thus expensive processors, which consume a large amount of power during the calculation process. For example, a known signal sequence of length 256 chips (in CDMA, a transmitted bit is also referred to as a chip) has to be determined for synchronization of the UMTS mobile radio system, which is currently being standardized. The sequence is repeated every 2560 chips. Since the mobile station is initially operating asynchronously with respect to the chip clock, the received signal must be oversampled to ensure that an adequate signal is still obtained even if the sampling position is poor. Owing to the sampling of the I and Q components, this leads to 256*2560*2*2=2621440 operations.